For environmental reasons, there is an increasing demand for lead-free replacements for lead-containing conventional alloys. Many conventional solder alloys are based around the tin-copper eutectic composition, Sn-0.7 wt. % Cu, and tin-silver eutectic composition, 96.5 wt. % Sn-3.5 wt. % Ag.
A ball grid array joint is a bead of solder between two substrates, typically circular pads. Arrays of these joints are used to mount chips on circuit boards.
The drop shock reliability of solder joints has become a major issue for the electronic industry partly because of the ever increasing popularity of portable electronics and partly due to the transition to lead-free solders. Most of the commonly recommended lead-free solders are high tin alloys which have relatively higher strength and modulus. This plays a critical role in the reliability of lead-free solder joints. Further, even though metallurgically, it is the tin in the solder alloys that principally participates in the solder joint formation, details of the IMC (intermetallic compound) layers formed with tin-lead and lead-free alloys are different. The markedly different process conditions for tin-lead and lead-free alloys also bear on solder joint quality. Brittle failure of solder joints in drop shock occurs at or in the interfacial IMC layer(s). This is due to the inherent brittle nature of the IMC, defects within or at IMC interfaces or transfer of stress to the interfaces as a result of the low ductility of the bulk solder.
There are a number of requirements for a solder alloy to be suitable for use in ball grid arrays (BGA) and chip scale packages (CSP). First, the alloy must exhibit good wetting characteristics in relation to a variety of substrate materials such as copper, nickel, nickel phosphorus, nickel boron (“electroless nickel”). Solder alloys tend to dissolve the substrate and to form an intermetallic compound at the interface with the substrate. For example, tin in the solder alloy will react with the substrate at the interface to form an intermetallic. If the substrate is copper, then a layer of Cu6Sn5 will be formed. Such a layer typically has a thickness of from a fraction of a micron to a few microns. At the interface between this layer and the copper substrate an intermetallic compound of Cu3Sn may be present. Such an intermetallic compound may result in a brittle solder joint. In some cases, voids occur, which may contribute to premature fracture of a stressed joint.
Other important factors are (i) the presence of intermetallics in the alloy itself, which results in stronger mechanical properties, (ii) oxidation resistance in multiple reflow, (iii) drossing rate, and (iv) alloy stability. This latter consideration is important for applications where the alloy is held in a tank or bath for long periods of time.